1. Field of the Invention
The present invention relates to television cameras and more particularly relates to television cameras provided with a biased light source device.
2. Description of the Prior Art
In television cameras there are generally used such image pickup tubes whose photoconductive surface (image pickup surface) is formed using PbO, As.sub.2 Se.sub.3, CdSe or other photoconductive material. However, these image pickup tubes have some problems in respect to response speed at rising response and afterimage by which the effective sensitivity of television the camera is limited. For example, when a moving object under a condition of low illumination is picked up by such image pickup tube, there is caused by the above mentioned unsatisfactory characteristics of rising response and after-image such unfavorable phenomenon that the picture appearing on the screen of the image receiving tube becomes vague or tailed.
It is known that the above problem can be solved to some extent by illuminating the photoconductive surface of the image pickup tube uniformly and at low illuminance using additional illuminating means particularly provided in addition to the imaging light of the object. This method is generally referred to as a bias lighting method and has been acknowledged to be a most effective method for reducing the difficulties of the known image pickup tubes.
As an example, U.S. Pat. No. 3,767,846 and U.S. Pat. No. 3,824,004 have disclosed television cameras comprising a biased light source device disposed outside of the light path extending from the picture taking lens to the image pickup surface.
In the bias lighting system hitherto used, the bias light is usually made incident upon the image pickup surface in two opposed directions to illuminate it as uniformly as possible. But, this manner of illumination can not assure a uniform illumination of the image pickup surface in all directions. Although the image pickup surface may be illuminated uniformly only in the two opposed directions, no uniform illumination is attainable with respect to the direction along the length of the biased light source device.